feat(player): add repeat off/all/one modes

The internal player handles setRepeatMode: repeat-one replays the
current track, repeat-all wraps the queue (and keeps a single-track
shuffle queue alive). The mode is broadcast in playback state,
persisted with the playback session, and toggleable from the Now
Playing transport row and the up-next sheet; a small shuffle toggle
joins the transport row for symmetry.

lib/providers/music_player_provider.dart

@@ -102,6 +102,9 @@ class MusicPlayerController {
     );
   }
 
+  Future<void> setRepeatMode(AudioServiceRepeatMode mode) async =>
+      _handler?.setRepeatMode(mode);
+
   Future<void> playNext(PlayableMedia item) async =>
       (await ensureInitialized())?.enqueue(item, playNext: true);